Method of making an electrical resistance device



April 2, 1963 J. S. HILL 3,083,445

METHOD 0F MAKING AN ELECTRICAL RESISTANCE DEVICE Filed Aug. e, 1957 UnitedStates Patent Office A 3,083,445 Patented Apr. 2, 1963 3,083,445 METHOD F MAKING AN ELECTRICAL RESISTANCE DEVICE James S. Hill, Cranford, NJ., assignor, by mesne assignments, to Engelhard Industries, Inc., Newark, NJ., a

corporation of Delaware Filed Aug. 8, 1957, Ser. No. 676,951 Claims. (Cl. 29-155.62)

The present invention deals with an electrical resistance device and more particularly with an electrically heated muifle for muffie type furnaces.

Electrical furnaces of the mufffe type comprise an insulated housing containing an elongated hollow muflle of suitable electrical insulating and heat conductive refractory material, such as Alundum, with an electrical resistance wire wound on the mule and a refractory cement, especially Alundum cement, covering the resistance wire.

When the furnace is operated at elevated temperatures, such as heat treating temperatures, especially at temperatures of the order of l400 C. and higher, the refractory materials, particularly the refractory cement, are subject to cracking whereby the winding is exposed and deleteriously affected. The cracking is particularly severe when the furnace temperatures are rapidly elevated and lowered.

For high temperature applications such as described above, the resistance wire employed is advantageously composed of the platinum group metals, for example platinum, and alloys of these metals, in view of the desirable resistance characteristics of such metals at high temperatures. Howevenin view of the cracking of the mufile and the refractory cement the platinum winding is exposed and the platinum volatili/Zes, whereby the electrical properties of the winding are no longer maintained constant.

It is an object of the present invention to provide an electrical resistance device for high temperature application. It is another object of the present invention to provide an electrically heated furnace mufe comprising a winding adapted to maintain substantially constant electrical operating characteristics. It is a further object of the present invention to provide a furnace muffle including a resistance Windin-g thereon which is covered by a coat of particular refractory material whereby the winding is effectively protected against volatilization. Other objects and advantages of the invention will become apparent from the description hereinafter following and the `dra-wings forming a par-t hereof, in which:

FIGURE 1 illustrates a longitudinal sectional view of a furnace mufile with an exposed winding thereon, and

FIGURE 2 illustrates a longitudinal sectional View of a furnace muflle with the winding coated and embedded according to the invention.

In accordance with the invention the resistance wires of the mule furnace, which are embedded in refractory material, are protected against volatilization and deterioration under high temperatures by means of a particular barrier coating between the metal resistance winding and the refractory material, the barrier coating itself being a particular refractory material, and the winding being cornposed preferably of a platinum group metal or alloys thereof.

Referring to FIGURE l, a tubular member 1 composed of a refractory material such as Alundum or zirconium oxide is covered over a major port-ion of its external surface area with a thin layer 2 of fused aluminum oxide which is deposited on the tubular member 1 while in the molten or fused state and which, on solidification, provides a dense coating integral with the said tubular member. A resistance winding 3 composed of a metal of the platinum group or an alloy composed of metals of the platinum group is cleaned Iand roughened by conventional means and wound over the coating 2. Having provided the winding 3 on the layer 2, the winding and at least a portion of the layer 2, is provided with a further coating 4 of the fused aluminum oxide which effectively combines with the layer 2 of the same material to embed the winding 3 within a -thin coat of the integrally combined layers 2 and 4 of fused and solidified aluminum oxide. Subsequently, the `coated winding is further coated with a layer 5 of Alundum cement or equivalent refractory cement to complete the furnace muflle. The muffle so constructed effectively protects the volatilization of the win-ding 3 should zany cracking occur in the refractory cement 5. It is of critical significance that the winding 3` is roughened prior to embedding in the combined layers 2 and 4 since the combined layers are substantially thin, of the order of between 0.001 to 0.03", to permit some expansion of the winding 3 without fracture of the embedding layers which are tenaciously mechanically bonded to the roughened windings. Without the embedding or barrier layer composed of the combined layers 2 xand 4, the Win-dings would be exposed to the Alundum cement the composition of which is, for example, parts A12O35 parts SiO2-5 parts CaO, whereby under heated conditions the SiO2 reduces in the presence of a platinum group metal forming silicon which reacts with the platinum group metal with ydeleterious electrical and mechanical effects causing erosion or volatilization of the metal and lowering the melting point thereof.

The fused aluminum oxide according to the invention is either aluminum oxide in a substantially pure state or aluminum oxide containing not more than five percent of at least one refractory oxide such as titanium dioxide, calcium oxide, zirconium .oxide and the like, with less than one percent SiO2, which amount of Si02 is of critical significance and below the deleterious Si02 content of the Alundum cement, whereby the fused aluminum oxide layer acts as an effective barrier agains-t the Si02 content of the Alundum cement and whereby the effective life of the winding is increased more than five times that of a winding in contact with the Alundum cement.

The layers 2 and 4, while shown separately as two layers divided by the dotted line 6, constitute in fact a single integral layer applied in two steps, the last applied fused layer 4 being bonded by fusion with the first applied layer 2.

In application, the layers are applied by feeding aluminum oxide powder into a hot flame, where the powder is fused, for example, by an oxygen-acetylene flame and, while in the molten or fused state, is sprayed onto the tube 1 and roughened Winding 3 substantially in the manner of producing fused quartz by melting and spraying quartz powder onto a fused quartz base which is well known in the art.

While the invention is particularly described and illustrated in connection with a muffle type furnace, it is intended to include herein electrical components such as resistance heated members which constitute a platinum group metal mounted on or contacting SiOz containing material which would normally be deleterious to the metal, and with the barrier layer hereinabove `described as a barrier layer between the metal and the support and which is within the scope of the appended claims.

What I claim is:

l. The method of making a resistance heated member comprising flame spraying an aluminum oxide first coating on a surface of a silica containing refractory support, mounting a metal taken Ifrom the Agroup consisting of platinum group metals and alloys thereof on the sprayed surface, land flame spraying a further coat-ing of aluminum oxide on the metal and said first coating thereby embedding the metal in fused aluminum oxide.

2. The method of making a resistance heated member. I

comprising flame spraying molten aluminum oxide to form a irst coating on a surface of` a silica containing refractory support, Winding arwir'e `cmpo'sed` of ai inet-alf selected from the 'group consisting of platinum groupmeta-lsand alloys thereof over the' sprayed surface, and further ame spraying moltenl aluminum oxide' on thek Wire and saidiirst coating'- therebyembedding the wire in fused aluminum oxide.

3. The method according to. claim 2 wherein the refractory support is :a furnace mule. Y

4. The method accordngfto claim 2 comprising applying a layer of Alundum ce-ment over the `aluminum oxideembeddin-g: thjei wire;

I minum. oxide embedding., said wirer is applied to provide a thickness Ithereof of from 0.001" to 0.03".

References Cited in the file of this patent UNITED STATES PATENTS 457,828 Morford' Aug. 18, 1891 1,096,414 Coolidge May 12, 1914 1,158,972 Boeck Nov. 2', 1915 1,298,421 Thomson Mar. 25,v 1919 1,802,892 Hansen Apr. 28, 1931 2,088,949 Feket'e' Y v 1 1 Aug. 3, 1937 2,661,385 Lincoln et al Dec. 1, 1953 2,961,335 Shepard Y A Y- Nov. 22, 1960 FOREIGN. PATENTS 6495398:k Gre-at, Britain -1 Jan. 24,. `1951 

1. THE METHOD OF MAKING A RESISTANCE HEATED MEMBER COMPRISING FLAME SPRAYING AN ALUMINUM OXIDE FIRST COATING ON A SURFACE OF A SILICA CONTAINING REFRACTORY SUPPORT, MOUNTING A METAL TAKEN FROM THE GROUP CONSISTING OF PLATINUM GROUP METALS AND ALLOYS THEREOF ON THE SPRAYED SURFACE, AND FLAME SPRAYING A FURTHER COATING OF ALUMINUM OXIDE ON THE METAL AND SAID FIRST COATING THEREBY EMBEDDING THE METAL IN FUSED ALUMINUM OXIDE. 